The invention relates to a process for the absorptive separation of NH3 and CH4 from a gas under high pressure (>50 bar abs.), which at least contains NH3, H2, N2 and CH4, hereinafter referred to as synthesis gas. NH3-rich synthesis gas is chiefly available in processes for generating NH3 from such synthesis gas, the conversion rate of said processes being really low because of the temperatures, pressures and catalysts that are applied, and the NH3 produced from synthesis gas having to be removed from a non-reacted gas stream. It is pointed out, however, that the invention is by no means restricted to this specific application.
Conventional plants for generating NH3 from synthesis gas are designed as loop systems operating at high pressure. Said configuration provides for the compression of the synthesis gas that contains H2, N2 and inert gas fractions, inter alia CH4, to a high pressure in the first step and then for the feed of the compressed gas to a reactor system in which part of the synthesis gas, i.e. 10 to 20%, is converted to NH3. The gas mixture obtained downstream of the reactor system is cooled with the aid of water such that as large a portion as possible of the NH3 formed condenses and can be withdrawn as liquid. In order to provide for the condensation of a further NH3 portion of the non-reacted gas mixture, additional cooling to much lower temperatures is required, hence an expensive refrigeration cycle. As it is indispensable to reduce the processing costs to an economic level, NH3 is separated only up to a residual content of approx. 4 molar % in the non-reacted gas stream.
In the case of a product concentration of 20 molar % at a synthesis pressure of 200 bar, for example, the dew point of NH3 is approx. 57° C. When providing a cooling by means of water, for instance, to 35° C., it is possible to reduce the NH3 content in the gas to 11.2 molar %, which permits a yield of 59% of the condensable product quantity. As the recycle gas fed to the reactor should have as low an NH3 concentration as possible, in this particular case 3.8 molar-%, it is common practice to install a low-temperature cycle downstream of the water cooling system so that further product amounts can be condensed at even lower temperatures (e.g. cooling to −10° C. to 0° C.).
Upon NH3 separation a purge stream is permanently withdrawn from the synthesis loop unit which prevents that the loop is enriched with gas fractions that are inert vis-à-vis the NH3-producing reaction as, for example, CH4 entrained by fresh synthesis gas. It is also necessary to recover residual NH3 and valuable fractions of the synthesis gas from the purge stream withdrawn, said fractions being re-compressed and then recycled to the synthesis loop. The loop is closed downstream of the purge stream withdrawal section by providing a circulator that compensates the pressure drop and by balancing the synthesis gas loss in the reaction through the admixture of fresh synthesis gas to the recycle synthesis gas.
But this system has the disadvantage that, for example, the NH3 separation at 180 bar synthesis pressure can be efficiently carried out down to a residual content of about 4 molar % only. In the case of a reaction system arranged downstream within a synthesis loop or fresh-gas reaction system of the NH3 separation unit, the said residual content will essentially equal the NH3 inlet concentration; the dilution caused by the intermediate admixture of fresh synthesis gas would change the NH3 inlet concentration to a minor extent only. Compared to synthesis gas that has no NH3 content, the NH3 inlet concentration of about 4 molar % would only permit a yield of about ⅘ of the NH3 amount recoverable per loop cycle.
Another disadvantage is that an expensive method is required to separate further NH3 from the purge stream withdrawn if its exploitation is not abandoned. The higher the purge stream rate, the larger the NH3 amount to be separated. But when the said rate is kept low, the inerts such as CH4 are enriched in the loop synthesis gas and their partial pressure reduces the yield obtained in the reaction system and the portion of NH3 that can be recovered with the aid of cooling water.
The criteria described in the previous paragraphs also apply to NH3 production plants that are not designed as loop systems because the NH3 portion not converted to synthesis gas will be exploited, for example, by downstream synthesis units. This also involves the need to separate as large an NH3 portion as possible from the synthesis gas downstream of the reaction system and to keep the inerts concentration low.
Hence, there has been a keen interest for years on the part of the chemicals industries to exploit by economic methods even small residual amounts of NH3 contained in the synthesis gas. A series of tests were carried out to remove NH3 by scrubbing; in most cases the solvent was an aqueous solution. This, however, involved on the one hand the problem to remove the dissolved NH3 from said solution, on the other hand the need to avoid volatilisation of fractions of the aqueous solution during scrubbing, said fractions entering the synthesis gas and thus causing technical problems in the downstream equipment, for example, poisoning of the catalyst. The said problems aroused the technological prejudice that there is not a safe and economic method to separate the NH3 from the synthesis gas by scrubbing. Moreover, there had been some interest in a selective removal of inerts, such as CH4, from the synthesis loop in order to reduce the necessary purge stream to a minimum.
It has also been described, for example, in German patent DE OS 1 924 892, that NH3 is absorbed from the gas mixture leaving the conversion zone, with the aid of a slowly evaporating organic solvent and that the absorbed NH3 is recovered upon solvent regeneration. Various alkylene glycol solvents have been suggested but in view of operational problems and related efficiency setbacks, said process has never achieved a breakthrough on the market for over 30 years. Patent WO 90/08736 A1 describes a further process of this type but on account of poor efficiency of this system in NH3 synthesis plants operated at a loop pressure of >100 bar, this process also failed on the market. A further process is outlined in DD 135 372 which provides for scrubbing to remove NH3 from off-gas or desorption gas with the aid of organic liquids such as ethylene glycol, di- or triethylene glycol or their mono- or dimethyl ether or mixtures thereof which may also contain up to 20% of water.
Hence, the aim of the invention is to overcome the said disadvantage and to provide a very efficient process suited to separate NH3 and CH4 from the synthesis gas irrespective of the operating pressure level.